Wiktionary's note says some people think this root may originally have referred to freshwater/inland seas, not oceans.

https://en.wiktionary.org/wiki/Reconstruction:Proto-Indo-Eur...

In many of the daughter languages it does regularly include saltwater connected to the world ocean (certainly the Latin "Mare Nostrum" for the Mediterranean!); it just seems uncertain from this description whether it necessarily had that sense in PIE.

That doesn't seem like "had no ... word for ... 'sea'" is a very clear or helpful description of the situation, though.